5.7.5.2.3.2 OSPF Area Range Configuration
After you configure an area identifier, you can specify a subnetwork address range that covers all the individual networks in this area. This technique limits the amount of traffic exchanged between Area Border Routers (ABRs) by allowing them to advertise a single summary range. By summarizing routes, the routing changes within an area do not have to be updated in the backbone ABRs or in other areas.
To optimize the route summary, first configure all the OSPF routers in an area so that they fall within a contiguous address range. The route summary consists of an address and mask, where the mask can be a Variable Length Subnet Mask (VLSM). Using VLSMs allows you to configure each subnetwork within a larger network with its own subnet mask. This provides a longer subnet mask that covers fewer host IP addresses, thereby reducing the size of the routing tables that have to be exchanged. (For more information on VSLMs, see RFCs 1219 and 1878.)
Parameter | Description |
Area Identity | An OSPF area that includes all the OSPF routers within the assigned address |
| range |
IP Address | The IP address used to calculate the area range. |
Address Mask | The subnet mask used to calculate the area range. |
Advertisement | Enables or disables advertising for this range. |
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